1. Field of the Invention
The present invention relates generally to navigational units, systems, and methods used in positioning applications, and in particular to inertial navigation units, systems, and methods for initialization, navigation, assistance, and correction, all in the field of inertial navigation and related applications.
2. Description of the Related Art
The present invention relates generally to units, systems, and methods of determining the location of mobile personnel and, particularly, to units, systems, and methods of determining the location of personnel working under specific conditions outdoors and/or within one or more structures. One type of navigating platform and architecture is an inertial navigation system (INS), which is a navigational aid that uses a computer (or controller) and certain sensors to continuously calculate, via dead reckoning, the position, orientation, and velocity of a moving object without the need for external references. An INS includes at least one computer (or controller) and a platform or module containing accelerometers, gyroscopes, or other motion-sensing devices. The advantage of an INS is that it requires no external references in order to determine its relative position, orientation, or velocity once it has been initialized.
Further, and with respect to inertial navigation systems, it is necessary to determine certain initial conditions for integration in the navigation routine. As is known, and upon power-up, an inertial navigation system has no knowledge of its attitude, velocity, position, or sensor biases. These quantities must be initialized to a best estimate of “truth.” In certain known applications, this initial estimate is an average of the first few measurements or, alternatively, simply set to zero. However, in many applications, proper and more accurate initialization of attitude is crucial.
In a strap-down navigation system (such as a system that uses a portable navigation unit attached to a portion of the user, e.g., the user's ankle, boot, leg, and the like), where the inertial sensors are fixed to the unit's reference axes (i.e., body frame), it is necessary to understand the mathematical relationship between the navigation frame and the body frame. This relationship may be defined as a 3×3 rotation matrix, or direction cosine matrix. Alternate representations include quaternion, Euler angles, and the Euler axis/angle.
One goal, which is addressed by the present invention, is the desire to mitigate error in the computation of the system attitude using the accelerometer specific force vector. Accordingly, the present invention at set forth hereinafter is useful in connection with any inertial navigation system that utilizes the accelerometer specific force vector to align to the local gravity vector as the primary means of alignment. Certain common alignment and estimation techniques and methods that are used in a variety of applications are described in Alignment of strapdown inertial navigation system: a literature survey spanned over the last 14 years, by Jamshaid Ali and Fang Jiancheng, Technical report, School of Instrumentation Science and Optoelectronics Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China and Fast quaternion attitude estimation from two vector measurements, by F. Landis Markley, Journal of Guidance, Control, and Dynamics, 25(2):411-414, 2002.
As discussed, attitude initialization is crucial in applications with limited or no aiding measurements. While certain aiding measurements can mitigate alignment error, mathematical constraints may prevent total convergence. Further, error in attitude degrades inertial navigation performance by introducing error in the integration of total acceleration, which requires knowledge of the system attitude, the local gravity vector, and Coriolis force. These errors compound into the velocity estimate, and further into the position estimate, such that error growth in position is parabolic in nature.
Due to the mathematical relationship between accelerometer biases, noise, and tilt error, it is difficult and often impossible to initialize the attitude precisely. Specifically, such measurements provide limited observability for use in estimating the tilt error in the attitude estimate. Further, no physical measurements of velocity, position, or attitude are available to aid the navigation algorithm or its various routines, and/or to effectively improve performance of the inertial navigation unit. Therefore, and in order to optimize performance, it is important to understand the “true” system attitude upon initialization.
In view of these and other shortcomings of known navigation and tracking systems, a need exists for new approaches to personal navigation and tracking. It is, therefore, desirable to develop improved units, systems, and methods for accurately determining the location of mobile personnel, which reduce the severity of or eliminate the above-described drawbacks and other problems with current location units, systems, and methods.